how to read acid base disorder

8/13/2019 How to Read Acid Base Disorder

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How To Read Acid Base

Disorder

Nanda, dr.Preodita, dr.

Irzan, dr.

Nova, dr.


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Stage I: Iden ti fy the Primary

Acid -Base Diso rder

Rule 1: An acid-base abnormality is present if

either the PaCO2 or the pH is outside thenormal range. (A normal pH or PaCO2 does

not exclude the presence of an acid-base

abnormality, as explained in Rule 3.)


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Rule 2: If the pH and PaCO2 are both abnormal,compare the directional change. If both changein the same direction (both increase ordecrease), the primary acid-base disorder is

metabolic, and if both change in oppositedirections, the primary acid-base disorder isrespiratory.

Example: Consider a patient with an arterial pH of7.23 and a PaCO2 of 23 mm Hg. The pH andPaCO2 are both reduced (indicating a primarymetabolic problem) and the pH is low (indicating

acidemia), so the problem is a primarymetabolic acidosis.


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Rule 3: If either the pH or PaCO2 is normal,

there is a mixed metabolic and respiratory

acid-base disorder (one is an acidosis and

the other is an alkalosis). If the pH is normal,

the direction of change in PaCO2 identifiesthe respiratory disorder, and if the PaCO2 is

normal, the direction of change in the pH

identifies the metabolic disorder.


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Example: Consider a patient with anarterial pH of 7.37 and a PaCO2 of 55mm Hg. The pH is normal, so there is amixed metabolic and respiratory acid-

base disorder. The PaCO2 is elevated,so the respiratory disorder is an acidosis,and thus the metabolic disorder must bean alkalosis. Therefore, this is acombined respiratory acidosis andmetabolic alkalosis. There is no primaryacid-base disorder in this situation; bothdisorders are equivalent in severity(which is why the pH is normal).


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Stage II: Evaluate Compensato ry

Responses

Rule 4: If there is a primary metabolic acidosisor alkalosis, use the measured serum

bicarbonate concentration in Equation 28.3 or

28.4 to identify the expected PaCO2. If themeasured and expected PaCO2 are

equivalent, the condition is fully compensated.

If the measured PaCO2 is higher than the

expected PaCO2, there is a superimposed

respiratory acidosis. If the measured PCO2 is

less than the expected PCO2, there is a

superimposed respiratory alkalosis.


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Example: Consider a patient with a PaCO2 of23 mm Hg, an arterial pH of 7.32, and a

serum HCO3 of 15 mEq/L. The pH isacidemic and the pH and PCO2 change inthe same direction, so there is a primarymetabolic acidosis. Equation 28.3 should

be used to calculate the expected PCO2:(1.5 15) + (8 2) = 30.5 2 mm Hg. Themeasured PaCO2 (23 mm Hg) is lowerthan the expected PaCO2, so there is an

additional respiratory alkalosis. Therefore,this condition can be described as aprimary metabolic acidosis with asuperimposed respiratory alkalosis


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Compensation for Metabo l ic Acidos is

Compensation for Metabo l ic A lkalos is


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Rule 5: If there is a respiratory acidosis or alkalosis, use

the PaCO2 to calculate the expected pH using

Equations 28.5 and 28.7 (for respiratory acidosis) orEquations 28.6 and 28.8 (for respiratory alkalosis).

Compare the measured pH to the expected pH to

determine if the condition is acute, partially

compensated, or fully compensated. For respiratoryacidosis, if the measured pH is lower than the expected

pH for the acute, uncompensated condition, there is a

superimposed metabolic acidosis, and if the measured

pH is higher than the expected pH for the chronic,

compensated condition, there is a superimposed

metabolic alkalosis. For respiratory alkalosis, if the

measured pH is higher than the expected pH for the

acute, uncompensated condition, there is a

superimposed metabolic alkalosis, and if the measured


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Example: Consider a patient with a PaCO2 of 23

mm Hg and a pH of 7.54. The PaCO2 and pH

change in opposite directions so the primary

problem is respiratory and, since the pH isalkalemic, this is a primary respiratory alkalosis.

The expected pH for an acute respiratory

alkalosis is described in Equation 28.6, and is7.40 + [0.008 (40 - 23)] = 7.54. This is the

same as the measured pH, so this is an acute,

uncompensated respiratory alkalosis. If the

measured pH was higher than 7.55, this would


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Respiratory

Acidosis

RespiratoryAlkalosis


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Thank You


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how to read acid base disorder

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